Digital camera modules continue to find more different types of platforms and uses. These include a wide variety of portable and wearable devices, including smart phones and tablets. These platforms also include many fixed and mobile installations for security, surveillance, medical diagnosis and scientific study. In all of these applications and more, new capabilities are being added to digital cameras. Significant effort has been applied to depth cameras as well as to iris and face recognition. A depth camera not only detects the appearance of the objects before it but also determines the distance to one or more of those objects from the camera.
There are several options for cameras to measure depth. There are passive systems that use multiple image sensors to determine the stereo offset between image sensors that are spaced apart from each other. Projectors are used in active systems to send coded light or structured light that is then analyzed by one or more image sensors. Structured light illuminates the scene with a specific pattern. The pattern is used to triangulate individually recognized projected features. Coded light projects a time varying pattern. Distortions in the pattern are used to infer depth. Other active systems use Time of Flight from a separate laser rangefinder or LIDAR as some examples. Active illumination is also used in various face, iris, and eye recognition systems.
Stereo imaging is easy to construct for consumer photography systems because it uses proven, safe, and inexpensive camera modules, but the stereo image is dependent on matching and comparing specific features in the scene. Clear sharp features are not always visible to the sensors, so active illumination is provided by a nearby LED (Light Emitting Diode) or other type of projector. This works well when the scene is not too bright, such as indoors or at night. In scenes with bright ambient light such as bright sunshine the active illumination may be overwhelmed by the ambient light.
To enhance the effectiveness of the active illumination, the pixels of the image sensors are operated in a way to improve their sunlight rejection of the camera. Pixel circuits have switches and capacitors that are operated to accumulate electrons generated by a photodetector in response to incoming light. The switches can also be used as a shutter to control the time at which an image is captured and the total amount of collected light at each pixel.